The present invention relates generally to plasma arc welding, and more specifically to a plasma arc welding device with coaxial wire feed for providing filler metal directly to the weld zone with high precision.
Filler wire is typically added during arc welding. Conventionally, filler wire is usually fed into the weld zone from the side via a wire feeder from a source such as a spool, through a conduit to the weld zone, and then aimed into the weld via a tip on the conduit. This mechanism is satisfactory for large welds where precise aiming of the filler wire is not required. However, the need to precisely aim the wire becomes critical in applications in which the weld pool is very small. In this case, the filler wire is correspondingly very small.
Present commercial/industrial arc welding technology employs a filler wire that is typically 0.030 inches in diameter or greater and a welding arc heat input that results in a large weld pool. The resultant weld is also large and is primarily useful for large weldments used in industry.
In order to fill a need for welding thinner materials and applications in which greater precision was required, a special wire feeder was developed as reported in U.S. Pat. No. 5,137,223, to Brandon et al. This invention was designed to feed small diameter (on the order of 0.010 inches) wire for miniature applications or for welding near heat-sensitive components. However, a high degree of precision is required to feed the wire into the small weld pool.
U.S. Pat. No. 4,321,454, to Willems et al. discloses the integration of the gas metal arc welding torch with a plasma arc in which the consumable electrode is coaxially fed. That device produces a high heat input arc weld that is not suitable for smaller welds in or near heat-sensitive components.
U.S. Pat. No. 4,924,053, to Morgan et al. discloses a gas tungsten arc welding device through which wire is coaxially fed. However, the tip of the tungsten electrode is located off to the side of the wire. This results in a change in bead characteristics depending on the orientation of the tip with respect to the direction of weld.
Accordingly, the need remains for a welding process that fills the gap between the small, high-energy-density laser and electron beam welds and the conventional, large arc welds, while providing filler metal via a self-contained, coaxial wire feed mechanism having high precision.
The present invention relates to a plasma arc torch apparatus with coaxial wire feed capability, comprising: a wire guide and a coaxial, hollow non-consumable electrode; wherein the wire guide directs filler wire through the tip of the hollow non-consumable electrode. The plasma arc melts the tip of the filler wire and deposits the molten material on the workpiece. Although the filler wire is fed directly through the center of the plasma arc torch and out through the center of the hollow electrode, the wire itself is not an electrode and does not carry any current. In this sense, the filler wire is non-electrified. Since the filler wire is fed coaxially through the center of the plasma arc torch""s electrode and nozzle, there is no need for additional equipment to position and feed the filler wire from the side. The present invention can provide uniform welds substantially no matter what direction changes occur during welding.